Merrick signed his card for 62 to rest at 11 under for the tournament, leaving out-of-form overnight leader Brendan Steele looking upwards.
Steele dropped two shots over his opening seven holes, leaving him five over for the tournament, although he will not have expected Merrick to perform as he did.
The 31-year-old did not drop a shot on his way around, with a second-hole eagle the precursor to eight birdies.
As he left the course, his nearest rivals were Swede David Lingmerth and the American pair of Hunter Mahan and Patrick Reed, all of whom were eight under at various stages of their rounds.
Tommy Gainey was nestled in on seven under, with Bubba Watson one of five Americans on six under. Trevor Immelman was alongside them.
Graham McDowell produced a round to rival Merrick's - the Northern Irishman settling with a seven-under 65 - but his poor opening 18 meant he was just three under overall.
Luke Donald was looking set to miss the cut, though, with England's former world number one even for the tournament after a steady one-under today.
Donald did miss out, becoming easily the most noteworthy casualty of the cut, with Ernie Els, Charl Schwartzel and Justin Leonard among those who scraped through on one under par.